The present invention relates to a vacuum chuck which is an attachment for holding a workpiece or tool in conducting machining or measurement utilizing a power of suction by means of a negative pressure of vacuum. More particularly, the invention relates to a vacuum chuck of which the suction head is formed of a unique material.
Conventional chucks for holding a workpiece or tool in machining or inspection include mechanical chucks, electromagnetic chucks, vacuum chucks and the like, of which vacuum chucks are used for holding a workpiece having a relatively small thickness and made of a non-magnetic material such as an aluminum-made disc for magnetic recording media, glass plate for photomasks, single crystal wafers of, for example, semiconductor silicon and the like.
A typical suction head in a conventional vacuum chuck has vacuum ducts in the form of perforations or in the form of grooved channels connected together including perforations. When the vacuum ducts are provided with perforations alone, the cross sectional area available for suction is so limited that the holding power of the workpiece is necessarily insufficient. When the vacuum ducts are formed of grooved channels, the pressure by which the workpiece is pressed against the suction head differs widely between the portion in direct contact with the groove and the portion not in direct contact with the groove so that the workpiece is more or less deformed to cause a problem in the accuracy of machining or inspection when an extremely high accuracy is desired.
With an object to solve these problems, it is proposed to use a sintered porous body of a metal or ceramic as a material of the suction head which serves to suck and attract the workpiece over the whole surface. A serious problem in these suction heads of a sintered porous metal or ceramic body is that, since metals and ceramics generally have a high hardness, workpieces made of a soft material such as aluminum are liable to be damaged by contacting with such a hard suction head of the vacuum chuck in the course of suction, holding and releasing. Moreover, self-excited vibration of the suction head sometimes takes place in working due to the high holding rigidity and low damping power against vibration to cause a difficulty in high-precision machining.
In this regard, conventional vacuum chucks for a workpiece of a soft metal are usually provided with a suction head made of a plastic and having grooved channels as the vacuum ducts. Such a plastic-made suction head is of course defective as is mentioned above because the workpiece attracted to the head is machined only insufficiently at the portions just above the grooves as a consequence of low rigidity leading to a poor accuracy of the flatness and shape after completion of the machining work.